1. Field of the Invention
The present invention relates to transferring gas directly into a liquid. In particular, it relates to a device which efficiently transfers gas into a liquid through a plurality of elongated tubular gas permeable membrane fibers without the formation of bubbles.
2. Description of the Prior Art
Gas transfer devices have a variety of applications, such as aeration for wastewater treatment and for improving water quality of lakes and reservoirs. It is desirable to minimize operating costs by having the most efficient transfer possible.
Pursuing the example of aeration, the major operating costs include the power required to pump air into the gas exchange devices and also the power required to pump liquid past the exterior of the gas exchange device. Although the prior art discloses a number of ways which attempt to make the transfer rate of gas more efficient through the use of different gas permeable membranes, operating difficulties have arisen. Hollow fibers having gas porous membrane walls and with the end remote from the gas source sealed have been used experimentally and have shown relatively high efficiencies but problems heretofore limited continuous operation. The use of sealed end fibers as gas transfer devices therefore has not developed despite the desired effect of efficient, bubbleless gas transfer from such fibers.
Wilderer et al. in an article entitled "Application of Gas Permeable Membranes for Auxiliary Oxygenation of Sequencing Batch Reactors," Conservation & Recycling, Vol. 8, Nos. 1/2, pp. 181-192 (1985) disclosed how they evaluated the effectiveness of a continuous flow of oxygen through the inside of silicon tubing for the oxygenation of waste water. They found that the transfer rate increases as the pressure of oxygen increases and the membrane thickness decreases. They also concluded that, in a wastewater aeration application, the transfer rate increases when a high concentration of oxygen is used. In addition, this high oxygen concentration is toxic to microorganisms, thus preventing them from colonizing on the membrane surface and reducing the oxygen transfer rate through the tubing wall.
In the investigations in the prior art, there has been a distinction made between a continuous flow system, that is, where oxygen will continuously flow through a hollow tube and out the remote end, as compared to a dead end or sealed end system, as shown herein. The dead end system is one where a tubular fiber having a gas permeable membrane wall is used and which fiber has the end remote from the gas inlet sealed. When pure oxygen is introduced into the fibers, and the fibers are in water, there is a back diffusion of gases such as nitrogen, water vapor, and carbon dioxide from the water to the fiber interior. As the oxygen passes outward through the walls of the membrane fiber, the concentration of water vapor, carbon dioxide and nitrogen inside the fiber increases. The greatest concentration of these species will exist at the end of the fiber that is remote from the gas inlet. Nitrogen and CO.sub.2 can and will diffuse back out through the fiber wall, when the internal pressure of these gases exceeds their external partial pressure. A steady state condition between the interior and exterior of the fibers will be reached in which there is no net accumulation of these gases within the fibers. However, as the concentration of water vapor increases inside the fiber, condensation occurs before the water vapor exits the fiber. This occurs when the internal pressure of water vapor exceeds the saturation vapor pressure. This phenomenon of condensation inside dead ended fibers has been observed in experiments by earlier investigators, but the solution to the problem previously involved either flushing out the gases that back diffuse and the water vapor, or stopping and emptying the membrane at periodic intervals. Continuous operation was not possible.
One prior investigation involved maintaining oxygen at a constant pressure inside a membrane bag, while water was pumped past the outside of the membrane. Since the oxygen did not continuously flow through the membrane bag, conditions for oxygen transfer to a liquid using a closed end fiber were approximated. Analysis of the gas inside the membrane after two weeks of use revealed that 60% of the gas was nitrogen and the investigators noted that the membrane bag should be emptied and refilled frequently to "maintain an oxygen partial pressure suitable for optimum transfer" and to sweep away the nitrogen that was transferred in. This investigation essentially taught that flow through the membrane was needed for efficient oxygen transfer.
Cote et al., in an article entitled "Bubble-Free Aeration using membranes: Mass Transfer Analysis", submitted for publication in the Journal of Membrane Science (1988) which acknowledged a prior art evaluated the continuous flow of oxygen through silicon rubber tubes to oxygenate waste water. Silicon tubes were used because they are non-porous and they may be operated at a high pressure before bubbles will form. By comparison the maximum operating pressure for no bubble formation in microporous polypropylene is very low. These authors also concluded that the only way a high oxygen pressure could be used with microporous fibers, without forming bubbles, (which decrease gas transfer efficiency), was if the water surrounding the tubes was also pressurized. They explicitly state that operation using closed end tubes without flow through is to be avoided because closed ends significantly decreased the oxygen transfer performance of the membrane and led to condensation of water vapor inside the tubes. The condensation was attributed to temperature changes. They did not recognize the back diffusion of water vapor and resultant phase change as the cause.
U.S. Pat. No. 4,181,604, issued to Onishi et al. (1980), discloses a hollow fiber membrane system for supporting a culture of and supplying oxygen to microorganisms that degrade organics in wastewater. Although Onishi notes that one end of each hollow fiber may be connected to a gas supply and the other end may be sealed and allowed to float free in the liquid, the emphasis of this disclosure is on creating an attractive environment for the microorganisms and providing a large surface area of the membrane to support the microorganisms. Onishi does not address the condensation problem associated with closed end fibers.
The prior art proposes a number of ways to maximize the gas transfer rate efficiency, such as using thin-walled membranes, high gas pressures, continuous gas flow, and pure oxygen. However, a practicable method of reducing cost by efficiently transferring a gas into a liquid has not been taught. One such method is to obtain a high transfer or utilization efficiency, i.e. to transfer most or all of the gas supplied to the fibers into the liquid. This high efficiency can be obtained by using dead end fibers and providing bubbleless gas transfer so that gas supplied to the fibers is not lost or wasted. However, until the present invention such fibers would periodically fill with water and become useless until emptied.
The present invention insures that condensation in hollow fibers will be discharged on a continuing basis so continuous operation is possible.